Conventional overlay measurement devices rely on an optical imaging device to measure the relative positional deviation between two sets of marks. Since the lens of an optical imaging device may be distorted, it is generally necessary to first move the marks to the center of the field of view of the optical imaging device and then detect the relative positional deviation between the two sets of marks using image recognition techniques, thereby eliminating the effect of lens distortion on the measurement results.
FIGS. 1A and 1B are plan views of a schematic representation of two sets of marks at the center of the field of view of an optical imaging device, as known in the art. As shown in FIGS. 1A and 1B, the position of a mark 102A in the current layer in the X-direction and the position of an etched mark 102B in a previous layer in the Y-direction can be measured using image recognition techniques so that the relative deviation of the positions between the two sets of marks can be determined.
However, in the process of placing the marks to the center of the field of view of the optical imaging device, there may be an error due to the placement of the substrate on which the marks are disposed, therefore, it is generally necessary to fine tune the placement to place the marks to the center of the field of view, and the process is time consuming. Further, there is also a time required to identify the scanned image, so that it takes a relatively long time to measure a set of marks (e.g., 0.5-1 second).
With ever decreasing size in technology nodes, the requirement for the set of measurements also increase and more sets of marks need to be measured. For example, if more than 60 exposure areas of a wafer are required to be measured, and assuming that each exposure area has five sets of marks, the measurement time will take about 5 minutes.
Thus, there is a need for a novel measurement apparatus and method to reduce the measurement time of the marks.